serial: 8250_dw: Use serial_lsr_in() in dw8250_handle_irq()

dw8250_handle_irq() reads LSR under a few conditions, convert both to
use serial_lsr_in() in order to preserve LSR flags properly across
reads.

Fixes: 424d7918 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt")
Fixes: aa63d786

 ("serial: 8250: dw: Add support for DMA flow controlling devices")
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Phil Edworthy <phil.edworthy@renesas.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220608095431.18376-6-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>